REV. B

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a

8-Bit ADC with Track/Hold Function

AD7821

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700

www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 2002

FUNCTIONAL BLOCK DIAGRAM

FEATURES

Fast Conversion Time: 660 ns Max

100 kHz Track-and-Hold Function

1 MHz Sample Rate

Unipolar and Bipolar Input Ranges

Ratiometric Reference Inputs

No External Clock

Extended Temperature Range Operation

Skinny 20-Lead DlPs, SOIC, and 20-Terminal

Surface-Mount Packages

GENERAL DESCRIPTION

The AD7821 is a high speed, 8-bit, sampling, analog-to-digital

converter that offers improved performance over the popular

AD7820. It offers a conversion time of 660 ns (versus 1.36

µs

for the AD7820) and 100 kHz signal bandwidth (versus 6.4

kHz). The sampling instant is better defined and occurs on the

falling edge of

WR or RD. The provision of a V

allows the part to operate from

±5 V supplies and to digitize

can be grounded and the AD7821 will operate from a single +5 V

supply, like the AD7820.

The AD7821 has a built-in track-and-hold function capable of

digitizing full-scale signals up to 100 kHz max. It also uses a

half-flash conversion technique that eliminates the need to gen-

erate a CLK signal for the ADC.

The AD7821 is designed with standard microprocessor control

signals (

CS, RD, WR, RDY, INT) and latched, three-state data

outputs capable of interfacing to high speed data buses. An

overflow output (

OFL) is also provided for cascading devices to

achieve higher resolution.

The AD7821 is fabricated in Linear Compatible CMOS

(LC

precision bipolar circuits with low power CMOS logic. The part

features a low power dissipation of 50 mW.

PRODUCT HIGHLIGHTS

1. Fast Conversion Time

The half-flash conversion technique, coupled with fabrication

on Analog Devices’ LC

version time. The conversion time for the WR-RD mode is

660 ns, with 700 ns for the RD mode.

2. Built-In Track-and-Hold

This allows input signals with slew rates up to 1.6 V/

µs to be

converted to 8 bits without an external track-and-hold. This

corresponds to a 5 V peak-to-peak, 100 kHz sine wave signal.

3. Total Unadjusted Error

The AD7821 features an excellent total unadjusted error figure

of less than

±1 LSB over the full operating temperature range.

4. Unipolar/Bipolar Input Ranges

The AD7821 is specified for single-supply (+5 V) operation

with a unipolar full-scale range of 0 to +5 V, and for dual-supply

(

±5 V) operation with a bipolar input range of ±2.5 V. Typical

performance characteristics are given for other input ranges.

5. Dynamic Specifications for DSP Users

In addition to the traditional ADC specifications, the

AD7821 is specified for ac parameters, including signal-to-

noise ratio, distortion, and slew rate.